Numerical Analysis of Suppression Effect of Asymmetric Slit on Cavitation Instabilities

In the present study, numerical analysis carried out around the cyclic flat-plate cascade with a asymmetric slit, so as to examine the suppressing or controlling effect of the slit for cavitation instabilities such as a cavitation oscillation which is similar to cavitation surge, and rotating cavitation. These instabilities cause various problem for the turbomachinery, for example, rotating cavitation causes an asynchronous axial vibration, and cavitation oscillation causes a pulsation of working fluid by resonance phenomenon of the system. Especially, in liquid propellant rocket engine, suppression process for these instabilities bring increase in cost of the launch. Therefore, it is thought that to find effective suppression technique is significant for turbomachinery. In this paper, two types of the flat-plate three blades cascade which have an asymmetric slit on each blade were analyzed, and compare with the result of cascade without slit. As a result, the cavitation oscillation is perfectly suppressed in both of two types cascade with an asymmetric slit. Also, in the one case of them, sub-synchronous rotating cavitation and rotating stall cavitation are suppressed. Furthermore, super-synchronous rotating cavitation which is not observed in single-stage cascade, is detected by arranging an asymmetric slit. These results indicate the possibility of suppressing cavitation instabilities or controlling the type of the cavitation instabilities by the arrangement of the slit. Moreover, the head performance at the almost same cavitation number is equal or slightly increasing by arranging the asymmetric slit.